What is Composting?

Composting is the controlled aerobic (oxygen-using) biological decomposition of moist organic (biologically derived carbon-containing) solid matter to produce a soil conditioner. Because it requires oxygen, it cannot be immersed in water (saturated).

The emphasis is on "controlled." This sets it apart from the uncontrolled decomposition that occurs in the natural environment: A leaf falls from a tree branch to the forest floor, and microbes transform it into a nutrient from that the tree can consume. The biochemical process is the same.

In a composting toilet, the objective is to transform potentially harmful residuals -- mostly human excrement -- into a stable, oxidized form.

The primary microorganisms responsible for composting are bacteria, actinomycetes and fungi. However, algae, mixomycetes (slime molds), viruses, lichens and mycoplasmas are other organisms present in the compost process. Soil animals, such as protozoa, amoeba, nematodes, earthworms and arthropods, also perform major roles by degrading surface litter, consuming bacteria and assisting in aeration.

What's So Good About Compost? "It is ironic that composting, so lately embraced in many economies, is one of the oldest forms of recycling known to humankind. As societies become reacquainted with this practice, its value as a natural solution to problems, from overflowing landfills to anemic soils, will become apparent. Then, with the proper institutional and economic incentives, composting could become as commonplace as the recycling of cans, newspapers, or paper is today." -- Gary Gardiner, Recycling Organic Waste: From Urban Pollutant to Farm Resource Organic materials (derived from life processes and have carbon in them) contain minerals, other chemicals and nutrients. However, applying raw organic materials directly to the soil is not the best way to use this organic matter and its nutrients. That's because when nutrients in materials are decomposing slowly, they are not available for use by plants. Also, decaying organic matter can tie up soil nitrogen that would otherwise fuel plant growth. That's the value of composting: It converts organic material into a stabilized product that builds soils and releases nutrients in a steady way. Composting can take place in a matter of hours or years -- it depends on the process factors described later. Essentially, bacteria and other organisms feast on carbon-rich matter and digest it, producing humus, a rich, stable medium in which roots thrive. Worked in soils, humus builds soil structure and provides a productive environment for plants and essential soil organisms. For our purposes, however, the primary value of composting is its ability to reduce waste. It also builds soils. Because it is riddled with pores, the humus in compost shelters nutrients ad provides extensive surface area to which nutrients can bond; indeed, humus traps three to five times more nutrients, water and air than other soil constituents do. These characteristics also help retain nutrients that could otherwise be leached or eroded away. In this way, adding organic mater to soils further reduces the need for additional nutrient applications. Compost has also been found to aid in suppression of plant diseases, often reducing or eliminating the need for fungicides. Compost releases plant nutrients gradually -- consider it a time-release vitamin pill. It holds moisture, and it's cheap. The nutrient value of composted yard wastes is modest, amounting to about 10 percent of the nutrients applied as fertilizer. The nutrient content of composted human waste, which is highly nitrogenous thanks to urine, is equal to that of many chemical fertilizers.

To Get More Technical...
Composting microorganisms produce enzymes that are extracellular -- like a chemical aura outside the organism's body. They transform molecules of organic matter into less complex chemicals and energy. All organisms have intracellular enzymes to manage the diverse complexity of the life pores. Enzymes are unstable proteins or protein-containing compounds that, when present in small amounts, promote a chemical reaction. The enzymes -- such as amylase, cellulose, lipase and protease -- are a few of the catalysts responsible for decomposition.

The solid, liquid and gaseous environment of the bioplex that forms a composting system provides a continuum of microniches for the organisms, and that affects the rate of biological transformation of organic matter

How fast something composts is affect by

1. environmental factors,
2. the composition and constituents of materials being composted,
3. the health and number of the organisms which are using the materials as a food source and
4. the management of the process by the operator.

Composted human waste from a BioLet NE chamber is emptied to be trenched into a flower garden in Sweden.

Composting is an "unsaturated" aerobic process. This means that the material being composted cannot be immersed in liquid, as that would fill the void spaces (the pores) in the composting mass, and prevent oxygen-carrying air from reaching the organisms digesting the food source. If the material becomes saturated, soon the remaining dissolved oxygen in the liquid is consumed. When there is no dissolved or free oxygen, that condition is said to be "anoxic." When the anoxic condition persists, "anaerobic" organisms, which cannot use free oxygen, will take over. The process is called "anaerobiosis," and is typified by offensive odors, such as that of rotten eggs, from sulfides, amines and mercaptans, and flammable methane gas produced by anaerobic bacteria.

Success Factors for Compositing Toilet Systems

The successful operation of a compositing toilet depends on several factors that must be maintained within certain broad ranges.

Beneficial Organisms and Available Nutrients
In a composting toilet, natural soil organisms act as living machines to decompose excrement into safe and valuable by-products. So, make sure there is a large population of bacteria, actinomycetes, fungi, yeast, algae, protozoa and other organisms.

To inoculate the composter with soil microbes, add two handfuls of sifted compost from a warm yard compost pile or successfully composted human waste. Sometimes a scoop of rotting leaves from the forest will do it, too. This will provide all the microbes for start-up. One gram of healthy soil may support 500 million bacteria, 20 million actinomycetes, 900,000 fungi, 100,000 yeast, 500,000 algae and 500,000 protozoa.

Or, you can purchase compost activators from a number of suppliers, including most garden centers.

How many and how effective they are depend on the environment provided, as well as pH and food supply. Human excrement and other organic material that you put in the toilet feed these microbes. The pH is self-regulating, if all of the other conditions are satisfactory.

Environmental Factors
Set the stage for the microbes to do the job. Variations in environmental conditions in the composter directly affect the various populations of organisms, increasing some and decreasing others.

The Aerobic Decomposition Agenda Below 42° F is biological zero -- little to no active bacterial processing takes place From 42° to 67°, you get psychrophilic (moldering) processing actinomycetes and fungi). From 68° to 112°, mesophilic bacteria are dominant. These are the typical composting toilet bacteria. From 113° to 160° thermophilic bacteria take over, and push the process to the limit.

1. Aeration

Take a deep breath. Let it out. Aahh. Your composter needs to do that, too, as the aerobes require free atmospheric or molecular oxygen. If there is an oxygen deficit (a state called "hypoxia"), the aerobes will die. They will be replaced with anaerobes (organisms that can exist only in the absence of molecular oxygen), which will slow the process and generate odors (thanks to their production of hydrogen sulfide, ammonia and amines) and potentially flammable methane gas.

The ventilation system in a composter should draw sufficient air across and through the ETPA.

A key factor, then, is the surface-area-to-volume ratio of the composting substrate (which includes the microbial population), because surface area allows direct contact with oxygen. If the volume of the composting material is greater than the surface area, then oxygen may not reach the microbes, and the process will be limited by this lack of oxygen. Mixing, tumbling, forced aeration and container design are ways composters provide a good surface-to-volume ratio.

To make the composting process work best, the materials being composted should have a loose texture to allow air to circulate freely within the pile. If the material becomes matted down, compacted or forms too solid a mass, the air will not circulate, and the aerobic organisms will die.

• Add bulking agents, such as wood chips, stale popped popcorn, etc., to increase pore spaces that permit air to reach deep into the biomass and allow heat, water vapor and carbon dioxide to be exhausted. Earthworms also create pores, as well as help break down wastes.
• Maintain adequate air flow through the material by proper ventilation (i.e., pressurized air, using convection or forced air by a fan) and/or by frequently mixing.
• Provide aerators, such as mixers, mesh, grates, air channels and screened pipes to help increase the surface area of the composting mass that is exposed to air.

However, too much air flow can remove too much heat and moisture -- make sure your composter is not too cool and dry as a result.

2. Moisture Content

The microbes in the composter need the right amount of moisture to thrive. Too much water (saturated conditions) will drown them, and create conditions for the growth of odor-producing anaerobic bacteria. In optimum conditions, the composting mass has the consistency of a well-wrung sponge -- about 45 percent to 70 percent moisture.

When the moisture level drops below 45 percent, it can become too dry for composting.

Also, excrement, toilet paper and additive will dry out but not decompose, thus prematurely filling the toilet (a good indicator that the mass is too dry). Dehydrating toilets are becoming popular in developing countries, but they do not stabilize the excrement, and that could lead to trouble.

If the moisture level is higher than 70 percent, leachate will pool at the bottom of the composter. You will have to drain it or evaporate it, because it will drown the microbes.

Urine and/or water from micro-flush toilets contributes most of the moisture in a composter, and may not be distributed evenly over the mass. (Note that urine-separating toilets are now available to reduce the leachate problem.)

Also, in composting toilets with heaters at the bottom, the upper parts of the biomass may become too dry.

In some systems, the drained leachate is resprayed over the top to prevent dehydration. This is not the best practice, as the concentrated salt and ammonia from urine are toxic to the beneficial bacteria and other organisms that are composting. Fresh rainwater (which has little or no dissolved minerals) is best for moisture control, but fresh groundwater from the tap (which contains significant dissolved minerals) will do as well. If the material is too dry, spray the compost mass with water, or add a cup of water periodically. Or, connect the dryer duct exhaust vent to the composter to contribute warm, moist vapor.

3. Temperature

The ambient temperature for acceptable biological decomposition is 78° to 113°F. Biological zero is 41° F (the temperature at which almost no microbial respiration occurs). At this temperature, most microbes cannot metabolize nutrients.

In most composting toilet systems, mesophilic (68° to 112°F) composting is at work. The heat generated by these microbes is usually lost through the vent stack, so composting toilets rarely reach thermophilic rates (113° to 160°F), which support thermophilic bacteria. This is the hot composting that takes place at the core of active yard waste composters (that's what generates the steam you see rising from the compost on a cold day). Achieving thermophilic composting would require either heating the composter -- which could be expensive -- or retaining the heat better by venting it less, which might mean odors and insufficient oxygen. In the highly contained environment of this kind of composter, it's a hard balance to reach.

Moldering toilets support psychrophilic organisms, whose optimum temperature is above 41°F and below 68°F. These are predominately fungi and actinomycetes bacteria such as Streptomyces griseus, which produces the antibiotic streptomycin. Moldering systems are sized much larger than mesophilic composting systems compensate for their reduced processing rate.

Moldering is also the last phase after mesophiulic and thermophilic processes have completed the early work of degrading sugars, fats and carbohydrates. As the process cools, fungi and actinomycetous bacteria slowly digest the cellulose and lignin in plant matter, such as wood chips and toilet paper.

Most small manufactured compost toilets have heaters and thermostats to maintain an internal temperature of 90° to 113°F to support the upper mesophilic composting range, while evaporating excess leachate. Evaporation of leachate tends to drop the temperature of the composter, through evaporative cooling.

Generally, the rate of processing in a biochemical system is directly proportional to the increase of temperature (within certain limits, the rate doubles with every 18°F increase). The warmer the process, the more capacity in a composter. The cooler the process, the slower the rate -- and more capacity may be needed for processing.

A composter in ambient 41°F will only accumulate excrement, toilet paper and additive until the temperature rises. That is why composter manufacturers state their capacities at 65°F (comfortable room temperature of an average human-occupied space).

In the earlier days of the composting toilet field, it was thought that the heat generated by the composting process would be enough to evaporate the leachate. (In fact, some optimistically saw the composter as a household furnace!) Alas, it was not to be, as much of this heat goes up the exhaust pipe.

For more on heat sources, see Chapter 4.

4. Carbon-to-Nitrogen (C:N) Ratio

While an important relationship to remember for aerobic bacteria nutrition is the carbon-to-nitrogen -- the "C:N" -- ratio, of the food source, its significance in composting toilets is often overstated.

Microorganisms require digestible carbon as an energy source for growth, and nitrogen and other nutrients, such as phosphorous and potassium, for protein synthesis to build cell walls and other structures (in the same way humans need carbohydrates and proteins). When measured on a dry weight basis, an optimum C:N ration for aerobic bacteria is 25:1.

Primarily due to the high nitrogen (from urea, creatine, ammonia, etc.) content and low carbon (glucose) content of urine (0:8:1), human urine has a low C:N ratio. Therefore, if the objective is to oxidize all of the nitrogen urinated into the toilet, this would require adding digestible carbonaceoous materials on a regular basis. (See the section on "Urine" in Chapter 4). However, the practical fact is that urine, which contains most of the nitrogen, settles by gravity to the bottom of the composter, where is is drained away or evaporated. In either case the nitrogen passes through the ETPA and is lost to the process! For that reason, adding large amounts of carbon will not help process the nitrogen, and will just fill up the composter faster.

The primary reason then to add carbon material is to create air pockets in the composting material (that's why some call carbon additive "structure material").

Digestible carbonaceous materials include carbohydrates (sugar, starch, toilet paper, popped popcorn), vegetable or fruit scraps, finely shredded black-and-white newsprint, and wood chips. A small handful of dry matter per person per day or a few cups every week is a good rule of thumb to maintain a helpful C:N ratio, absorb excess moisture and maintain pores in the composting material. For more on additives, see Chapter 6.

5. Process Control

Process control is how you optimize the composting process by controlling the external variables that affect the process, as described above.

The following are some process controls that may be in a composter:

• Motorized and manual mixing or turning provides aeration and moves microbial communities into contact with unprocessed ETPA.
• Blowers and fans remove odors and gasses that are the by-products of composting, such as carbon dioxide and water vapor. Fan-speed controllers optimize the efficiency of the fan
• Heaters maintain optimum temperature of the microbes and evaporate the water from leachate.
• Pumps are used to move leachate to a management system. Others are used to spray water over the mass.
• Warning indicators and alarms tell the manager when something needs attention.

Some compost system designers are integrating sensors and microprocessors that trigger and manage these process controls (so you don't have to). These are common in larger processing plants for municipal solid waste, sludge biosolids, and agricultural residues; they monitor temperature, carbon dioxide production, and oxygen and moisture content of the composting mass. Some of these are being integrated into AlasCan composting systems. Harvard University designed a prototype of such a composter (but it is still far too expensive for production). Small manufactured composters have yet to become so sophisticated. But it is just a matter of time before these sensors and automatic process controls are part of most commercial composting toilets.